The disclosure is directed to extending the dynamic range of analog optical receivers of the type used in Hybrid (optical) Fiber/Coax (HFC) return path systems such as used in cable television. Due to system characteristics, the optical receivers in such systems often face a large variation in the optical input power requirement, that results in an even larger variation in the RF gain requirement. The conventional solution is to add attenuation to reduce the excessive gain. This is a simple and straightforward approach and works well when the needed attenuation is relatively small. However, when the amount of needed attenuation is close to the gain of the amplifier, the added attenuation produces unwanted noise and distortion.
In a HFC system such as a cable television or other communications network, the “forward path” (head end to subscriber) media (optical fiber and coaxial cable) carries modulated video, data, and cable telephony signals. In the return path (subscriber to head end), the media carries mainly data signals in the QPSK or QAM modulation format and frequency division multiplexed within the frequency bandwidth of interest, e.g., 5 to 42 MHz. The return path data signals originate as electrical signals from cable modems at various subscriber locations, are combined at an optical node, and transmitted therefrom by an optical transmitter. The corresponding optical receiver converts the transmitted optical signal into an electrical RF (Radio Frequency) signal and then amplifies the RF signal to the desired output level. The RF signal is then combined with other optical receiver RF output signals and transmitted to a CMTS (Cable Modem Termination System) input and ultimately to the system head end.
Because of the nature of a HFC system, the optical receiver often faces a large variation of optical input power depending on the optical transmitter optical output power, link loss, the number of receivers combining signals at the CMTS input, and the CMTS input power requirement. The optical power received by an optical receiver can be as low as −15 dBm and as high as +2 dBm. For every 1 dB of optical power variation, the corresponding RF signal varies by two dB due to the optical receiver's optical to electrical conversion process in its photodetector. For optical power varying 17 dB (from −15 dBm to +2 dBm) the RF signal will vary by 34 dB. This is a very large gain variation. Therefore the optical receiver must be capable of providing RF signal gain over a broad range. In addition, sometimes a certain degree of optical AGC (Automatic Gain Control) is desirable in order to maintain the signal stability and additional gain must be preserved to allow the AGC to take effect. This adds further gain variations.
In a HFC network return path, a key system performance parameter is the NPR (Noise Power Ratio) dynamic range. NPR is the ratio of carrier magnitude to unwanted noise magnitude. For a return path to operate seamlessly, the system NPR must meet a minimum requirement, otherwise errors occur and system transmission speed slows down. If the NPR is plotted on a y-axis of a graph and the system input or output power is plotted on the graph x-axis, the resulting curve usually has a reverse V shape. The dynamic range is the dB difference between the two input or output power points at which the NPR is identical.
Dynamic range thus is essentially the “headroom” at which the return path operates. The headroom is required because in the return path there are various unpredictable sources of noise interference such as impulse noise and ingress (external) noise. The larger the dynamic range, the better is the system performance in dealing with unpredictable noise interference. The dynamic range defined here is that in which the low end of carrier performance is limited by CNR (carrier to noise ratio) and the high end is limited by second or third order distortion. On NPR plots, the noise contribution to the left side of the reverse V shape curve is thermal noise and the noise contribution to the right side of the curve is distortion noise. The distortion noise is due to system or device non-linearity. If the carrier signal is a CW (continuous wave) tone, the distortions are manifested as second and third order harmonics. Since the return path signal carriers are in the form of QPSK and QAM, their power spectral densities are alike in terms of thermal noise and therefore its harmonics are also alike in terms of thermal noise.
In a typical system, the NPR is predominately the optical transmitter NPR in the middle of the optical power range received by the associated optical receiver. However, at very low and very high optical power, the optical receiver's NPR dominates the system's NPR. At very low input optical power, the optical receiver thermal noise contributes the majority of the noise, and at very high optical input power the optical receiver distortion noise contributes majority of the noise. In the optical receiver, noise contributions in terms of components are from the photo-detector and RF amplifiers. Commercially available analog photodetectors are well behaved in both respects such that additional improvement in this area requires substantial component cost increase with little or no improvement of overall system performance. RF amplifiers, on the other hand, are available with various performance tradeoffs between gain, noise figure, distortion and power consumption. Generally speaking, one skilled in this field will have little difficult choosing the amplifiers having the best tradeoffs. What remains is the gain and attenuation approach to obtain optimum performance for both extreme (high and low) power conditions.
There are known approaches to solve this in the field of digital communications where the signals are base band signals operating between two (digital) logic states. These approaches are not applicable to analog optical receivers as in cable television systems, because many performance requirements critical to analog optical receivers are not critical to digital optical receivers. For example, transimpedance amplifiers used in digital communications inherently have low performance in terms of distortion. Also, impedance matching and frequency response are not critical in digital communications but important in an analog receiver. Therefore in analog systems in order to support large power variations of the gain portions for an optical receiver, matched attenuators are connected between the RF amplifier stages, see FIG. 1.
FIG. 1 shows a receiver the primary optical element of which is photodetector (phototransistor or photodiode) D1 12. This is conventionally arranged so that it receives light, as indicated, from e.g. an optical fiber, via a lens (not shown). Diode 12 is coupled to a voltage source V with filter capacitor C1 14 and the diode 12 output current is coupled to a transformer T1 16. Blocking capacitor C2 18 couples the output RF signal from the transformer T1 (but blocks any D.C. signal) to the first of a series of amplifier stages which in this case includes an RF amplifier A1 20 and a variable attenuator P1 22. Each of the two subsequent stages respectively also includes an amplifier and variable attenuator 24, 26; 28, 30. The attenuators provide a constant impedance (at any one setting). The final stage includes only the amplifier A4 32 providing the RF output signal. The variable attenuators 22, 26, 30 are each controlled, via its control terminal, by a control circuit 36 which senses the level of output power from photo-detector D1 across resistor R1 34.
The number of amplifiers used depends on the gain required and the gain of each amplifier. In this configuration, the noise performance of amplifier A1 makes the most noise contribution while amplifier A4 makes the most distortion contribution. The operating points for amplifiers A1 and A4 are set by the system requirements and after the particular amplifiers are chosen, a base line for the achievable maximum dynamic range is set. The dynamic range performance further degrades when the inter-stage amplifiers' (A2, A3) noise and distortion contribution become significant. Tradeoffs must be considered in order to minimize the contribution by the inter-stage amplifiers. When the RF signals vary in strength as much as 30 dB, the inter-stage amplifier contribution will unavoidably be significant. Generally speaking, more stages allow better thermal noise and distortion performance tradeoffs. In the case of low optical input power, the attenuation is allocated to attenuators P2 and P3 with zero attenuation at attenuator P1 in order to minimize the noise contribution by amplifier A2. In the case of high optical input power, the attenuation is distributed among the three attenuators P1, P2, P3 where attenuator P3 has the lowest attenuation in order to minimize the distortion contribution by amplifier A3.
In order to illustrate this, assume the amplifiers in FIG. 1 are all identical and each has a gain of 15 dB. Further assume that for a given set of conditions, when optical input power is −15 dBm, all attenuators P1, P2, P3 must be set to 0 dB attenuation to achieve required RF output power. If the same RF output power must be maintained when the optical input power is changed to 0 dBm, then 30 dB attenuation is required in the signal path. If the 30 dB attenuation is evenly distributed among attenuators P1 and P3 (that is, 15 dB each) while the attenuation of attenuator P2 is set to zero, then amplifier A2 will make exactly the same thermal noise contribution as does amplifier A1, since the input power to these two amplifiers is identical.
By the same token, amplifier A3 will make exactly the same distortion contribution as amplifier A4, since the output power of these two amplifiers is identical. When this occurs, the thermal noise degradation is 3 dB while second order distortion degradation is 3 dB and third order distortion degradation is 6 dB. These are significant degradations for receiver performance and when the receiver is used in a system, the overall system NPR performance will be degraded. In this example, a 3 dB receiver thermal noise degradation may not be a significant degradation for the system since the transmitter thermal noise is still dominant, but 6 dB receiver distortion degradation can be significant. If the trade off is made to reduce the distortion but increase thermal noise, then the thermal noise contribution by the receiver will take effect. Of course, if the attenuation is distributed among all three attenuators, the degradation will be reduced but this will not be significant, and so this requires a more elaborate attenuation scheme.